The ITU recommendation ITU-T O.201, which is incorporated by reference herein, describes Q-factor measurement equipment, commonly also referred to as a Q-monitor. A Q-monitor is a powerful circuit for performance monitoring of high speed optical signals as well as a sensor for adaptive adjustment of decision threshold, decision phase and other parameters like amplifier gain or equalizer setting in an EDC (Electronic Dispersion Compensation) based receiver.
The basic Q-monitor circuit shown in FIG. 1, which is taken from ITU-T O.201, uses two decision channels, where one channel is operated in the optimum sampling point (regenerated input signal), whereas the other scans the input signal in the amplitude/phase dimensions. The outputs of the two decision channels are compared (EXOR) and integrated (error counter) for different monitor thresholds and phases. This results in a one or two dimensional eye contour. If an algorithm is used to derive from this measurement the optimum decision threshold and phase, the receiver is operated in its optimum sampling point and additionally computes a Q-factor for performance monitoring purposes.
Q-factor measurement is an established method for characterization of optical channels. Particularly at low bit error rates the method has the advantage of taking less time than a traditional BER measurement, which requires to count bit errors over a statistically significant time period. The Q-factor is defined as the (electrical) signal-to-noise ratio at the decision circuit of a digital signal receiver.
A Q-monitor is also described in EP 0 923 204 A2, which is incorporated by reference herein. EP 1 445 879 A1, which is likewise incorporated by reference herein, modifies the basic Q-monitor concept in such a way that the two decision channels can be alternatively operated in monitor and data sampling mode. A crossover switch following the two decision channels selects the appropriate decision channel for the data and the monitor paths. This way, all phase and amplitude offsets in the circuit can be compensated effectively.
Existing Q-monitors are, however, not well suited for highest bitrate applications at well above 10 Gbit/s. Moreover, for further processing of highest bitrate signals (i.e., overhead processing, FEC etc), such signals are typically converted to a parallel format.
It is therefore an object of the present invention to provide a digital signal receiver with Q-monitor, which is suited for highest bitrate applications, preferably at 40 GBit/s and above.